The study of human cells for medical screening, diagnostic and other medical purposes is well known. For example, red blood cell count, mean cell volume (MCV), hemoglobin content and hematocrit are well known and commonly employed red blood cell parameters used in medical study and patient care. It is typical to free the hemoglobin from the red cells by lysis, i.e., destruction of the cells. Such cell destruction is accomplished by dissolving or rupturing the stroma of the cell and also is termed hemolysis.
It is well known that the osmotic pressure of solutions, i.e., their osmolality, such as saline solutions, varies with their concentration and type of solutes, and that the difference between the osmotic pressure within a cell and that of its surrounding environment causes the cell to change in volume and electrical resistance.
The so-called physiological saline solution is isotonic and cells suspended in such a solution will neither swell nor shrink in size or volume, nor will their electrical resistance change. Distilled water is hypotonic with respect to the isotonic solution in which blood cells reside within the human body. Saline solutions can be isotonic, hypotonic or hypertonic, depending upon their concentration.
A relatively well known, but less often employed, red cell test is osmotic fragility, which is a measurement of rate of the hemolysis of the cell under controlled changes of osmotic pressure. The Fragiligraph is an instrument which measures osmotic fragility and, over a period of approximately five minutes, hemolizes an increasing number of red cells in a sample, optically measures the amount of released hemoglobin, and plots a characteristic ogee (S shaped) curve or its derivative. The abscissa of the curve is time and the ordinate is the amount of hemolysis. A few variations in curve shape have been recognized and related to different health conditions.
It is necessary to understand the relationship between osmotic pressure and hemolysis to be able to appreciate that the present invention is based upon a treatment and measurement of the cells which does not destroy the cell stroma and thus differs from hemolysis.
Cell and particle counting and measuring instruments, examples being those sold under the trademark Coulter Counter.RTM. by Coulter Electronics, Inc., Hialeah, Fla., employ electronic sensing means which directly respond to the electrical resistance of each cell to count and measure each cell and progressively record cell parameters of a sample of cells in an isotonic solution. The Coulter Counter.RTM. particle measuring instruments operate upon the well known and well documented principle of particle and cell measurement employing a sensing aperture path, which also is disclosed in Coulter U.S. Pat. No. 2,656,508 and improvement U.S. Pat. No. 3,259,842. A form of MCV measuring apparatus especially useful with a Coulter Counter.RTM. instrument is taught in U.S. Pat. No. 3,473,010. The response of a Coulter Counter.RTM. electric sensor is influenced at least by the shape, deformability and flow rate of the microscopic item being measured as it flows through the sensing aperture path. Since most cells are subject to some deformation as they pass through the sensing aperture path, their electrical resistance measurement and the recorded measured volume may differ from their true volume. To distinguish between true volume and measured volume, the term "apparent volume" will be employed herein to refer to measured volume.